Photosystem II (PSII) plays a key role in water-splitting and oxygen evolution. X-ray crystallography has revealed its atomic structure and some intermediate structures. However, these structures are in the crystalline state, and its final state structure has not been solved because of the low efficiencies of the S-state transitions in the crystals. Here we analyzed the structure of PSII in solution at 1.95 Å resolution by single-particle cryo-electron microscopy (cryo-EM). The structure obtained is similar to the crystal structure, but a PsbY subunit was visible in the cryo-EM structure, indicating that it represents its physiological state more closely. Electron beam damage was observed at a high-dose in the regions that were easily affected by redox states, which was reduced by reducing the electron dose. This study will serve as a good indicator for determining damage-free cryo-EM structures of not only PSII but also all biological samples, especially redox-active metalloproteins.

中文翻译：

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光系统的高分辨率低温电磁结构II：电子束损伤的影响

光系统II（PSII）在水分解和氧气释放中起关键作用。X射线晶体学已经揭示了其原子结构和一些中间结构。然而，这些结构处于结晶态，并且由于晶体中S-态转变的效率低，所以其最终态结构尚未解决。在这里，我们通过单粒子冷冻电子显微镜（cryo-EM）以1.95Å的分辨率分析了溶液中PSII的结构。获得的结构类似于晶体结构，但是在冷冻EM结构中可见PsbY亚基，表明它更紧密地表示其生理状态。在容易受到氧化还原状态影响的区域中，在高剂量下观察到电子束损坏，通过减少电子剂量可以减少电子束损坏。